Gas chromatography (GC) is a well known method of separating mixtures of chemical compounds. A sample mixture to be analyzed is introduced into a chromatography column. Separation of the mixture occurs because the components travel through the column at different rates, eluting at different times. The period of elution of a component is commonly referred to as a "peak." Capillary columns used in modern GC provide very high resolution in sample separation because they provide very narrow peaks, often as narrow as one second.
A variety of methods and devices are available for qualitatively and quantitatively detecting the sample components eluting from the column. It is often desireable to use a mass spectrometer as part of the detection apparatus because of its ability to accurately identify an extremely wide range of substances. One type of mass spectrometer relies on the principles of ion cyclotron resonance (ICR).
Typically an ICR device comprises a sample cell wherein the sample is ionized and subject to a unidirectional uniform magnetic field. An oscillating electric field having one or more frequency components is applied in a direction orthogonal to the magnetic field. From the well-known equation, EQU (.omega.B=)e/m (1)
(where .omega. is the frequency of the oscillating field, B is the magnetic field strength and e/m is the charge-to-mass ratio of the ion), it is seen that ions will resonate at particular frequencies in a given magnetic field. The resonant ions will absorb the rf energy and will be accelerated in roughly circular planar orbits of increasing diameter perpendicular to the magnetic field. A signal current is then detected as resonant ions strike and/or induce currents in the detector electrodes. Examples of ICR devices are described in U.S. Pat. Nos. 3,390,265; 3,461,381; 3,742,212; 3,505,517; 3,937,955; and 4,464,570.
Since each species of ion resonates at a particular frequency, it is necessary to subject the sample to a range of frequencies to detect the various chemical species present. The simplest approach to accomplish this is to sweep a frequency generator through a range of frequencies. However, this approach takes too long to be useful in GC/ICR.
The technique utilized to date in GC/ICR for establishing the mass spectrum of the chemical species present in the sample consists of wide-band RF excitation followed by Fourier transformation (FT) of the resulting transient decay time domain signal into the frequency domain. This approach permits the excitation of all the ions at once, eliminating the need for a time-consuming frequency sweep. Once the FT has been made, it is a straightforward task to calculate the mass spectrum of the ions within the sample cell. Any given frequency peak corresponds to a particular ion in the sample cell at the time of excitation. Moreover, the magnitude of the frequency peak is a function of the quantity of the ion in the cell. To obtain quantitative information about a particular chromatographic peak, it is necessary to integrate the detector response a multitude of times during the elution of the peak. In practical terms, at least 10 points are necessary for reasonably accurate integration.
In practice it has been found that the transient decays in the time domain should be digitized at twice the maximum resonant frequency encountered in the sample to prevent aliasing and to provide good mass resolution. By straightforward calculation, it can be shown that in a magnetic field of one Telsa, a range of ions with mass between 20 and 600 atomic mass units (amu) exhibit resonant frequencies in the range of 768 KHz to 25.6 KHz.
After performing the FT analysis of the digitized time domain signal, the resulting mass spectrum must then be compared to the mass spectra of known standards to confirm the presence of particular substance. Both the FT calculation and the subsequent comparison to known standards are routinely performed by computer. These operations require extensive data manipulation and take a relatively long time making it difficult to provide a real time output signal. This is particularly a problem with gas capillary chromatography where the peaks are narrow making it very difficult to obtain a sufficient number of data points needed for accurate integration.